The weight of 8 inches of snow on the roof combined with a weak weld holding together a truss seat caused the Feb. 13 collapse of Blacksburg High School's gymnasium roof, according to a final report from forensic engineers Forcon International released this afternoon.

The welds at the truss seat -- the piece that connects the corner of a roof truss with a support column -- did not meet American Welding Society’s standards, causing one of two main trusses that held up the roof over the 152-by-106-foot gym to buckle, the report says.

No one was injured in the collapse, but since then the entire school has been closed indefinitely. Students are scheduled to attend Blacksburg Middle School in the upcoming academic year. Those middle school students will attend classes at the old Christiansburg Middle School.

Forcon’s preliminary report released June 11 ruled out an overload of snow as the primary cause. Building codes at the time the gym was built, 1974, required the roof to be able to support 35 pounds per square foot, the preliminary report said. Even though Blacksburg had more than 50 inches of snow this winter, only 8 inches were on the roof when it collapsed. That generated about 23 pounds per square foot, it said.

The collapse began where the west main truss connected to its support column to the left of the front entrance to the gym. According to the report, that truss was "rotating down and to the right" until it broke loose from the column. That caused the east main truss to snap from its column on the other side of the front entrance, and that pulled both trusses loose from the columns on the school side of the gym.